Travelling wave tubes



July 5, 1955 L.. M. FIELD TRAVELLING WAVE TUBES Filed June 50, 1950 INVENTOR L ESTER M. F/EL D j ATTORNEY United States Patent I TRAVELLHNG WAVE TUBES Lester M. Field, Palo faite, Calif., assigner to The Board of Trustees of the Leland Stanford Junior University, Stanford University, Calif., a legal entity having corparate powers of Caitornia Appiicaticn .lune 3i?, la-54), Serial No. 171,479

Claims. (Cl. 31E- 3.6)

This invention relates to travelling wave tubes, and more particularly to improvements in devices of the type wherein an electron beam is directed along a slow wave propagating structure, such as a helix, in energy exchanging relationship with a wave travelling thereon.

Usually in the design and operation of such tubes, compromises must be made between various factors which atleet the maximum usable gain, bandwidth, and the noise figure. Losses in the helix tend to reduce the gain, and thus a low-loss helix is desirable to achieve high gain. However, the use of a low-loss helix permits some of the amplified signal to travel back down the helix and reenforce the input signal at certain frequencies, and this will cause the ampliiier to oscillate.

The backward transmission, and hence the tendency to oscillate, may be minimized by breaking the helix into two sections, forming a gap which interrupts the transmission along the helix. The signal is carried across the gap by the electron stream. In theory, as substantiated by actual measurement, the reduction in over-all gain introduced by such a gap is about 3.5 db.

In a practical device intended to provide amplification throughout a substantial bandwidth, the two helical sections must be terminated properly at each side of the gap so that no reflections of energy can occur. Broad band terminations require the use of tapered attenuation, in which the signal to be absorbed is dissipated gradually over a more or less extended path, rather than abruptly. Distribution of the attenuation over the entire length of the helix, or a substantial portion thereof, as by using high resistance wire, introduces very large loss in the forward gain (about of the reduction in backward transmission) and consequently is not desirable.

Heretofore it has been the practice to concentrate the attenuation in a tapered section near the middle of the tube, by coating a part or the glass envelope with resistive material such as carbon. However, it isfound that this also reduces the forward gain by essentially the same amount as did distributed attenuation.

At least part of the explanation for this phenemenon appears to be that the usual center attenuator needs to be around ten wavelengths (axially, in the helix) long, and this region acts as a drift space, allowing considerable debunching of the electron stream. Moreover, this space amounts to one fourth to one third of the length of the helix, and is wasted as far as providing amplification is concerned. Furthermore, such a drift space saturates at a much lower power level than the rest of the tube.

In view of the foregoing facts, it has been necessary heretofore, in order to realize substantial gain without oscillations, to use such high beam currents, that partition noise could not be avoided. One of the important factors aiecting the noise iignre is the transit angle from the anode or accelerator electrode or other gun elements to the beginning of the helix. For lowest noise figure in the tube, this transit angie is often required to be as small as possible.

lt is also necessary, however, to provide means for 2,712,614 Patented July 5, 1955 putting the signal to be ampliiied onto the helix. The

v prior art practice in this respect is to make the initial portion of the helix of greater pitch than the main body of the helix, stretching out the tirst few turns to provide a gradual transition from the wave guide or transmission line input means to the helix. This and other similar expedients require axial space in the tube at the critical transit angle region, and thus require a choice between optimum design for noise figure and gain on one hand, and the use of previously known matching devices on the other.

Similar considerations arise in the design of grid-helix travelling wavetubes, wherein the input signal is put on the electron stream by means of a control grid. Here the distance from the grid to the helix must be kept as short as possible to prevent deleterious long transit angle etlects such as space charge debunching, and at the same time the front end of the helix should be terminated to prevent reection and avoid oscillation.

One of the principal objects of the present invention is to provide improvements in travelling wave tubes to enable the design of such tubes to provide both high gain and low noise figure.

Specifically, it is an object of this invention to provide improved broadband termination and impedance matching devices for travelling wave tubes and the like, occupying a minimum of axial space.

Another object is to provide gap attenuators for travelling wave tubes, which are broadband by virtue of including radially extended dissipation regions, but which are axially short so as not to include any substantial drift region.

This invention will be described with reference to the accompanying drawings, wherein:

Fig. l shows a longitudinal section of a helix type travelling wave tube incorporating a center gap attenuator and input and output matching devices in accordance with the present invention,

Fig. 2 is a transverse section the Fig. l,

Fig. 3 shows a longitudinal section of the input end of a grid input helix tube embodying a low rellection termination according to this invention at the input end of the helix, and

Fig. 4 is a sectional View of the input end of a resonator input helix tube embodying the invention.

Referring to Fig. l, the travelling wave tube depicted therein includes an electron gun comprising a cathode assembly 1 and an anode or accelerator electrode 3, a conductive helix divided into two serially arranged sections 5 and '7, and a collector electrode 9. All of the fore-` going elements are enclosed in an evacuated envelope 1l. Terminal wires 13, 15 and 22 are connected to the cathode 1, accelerator 3, and collector 9 respectively, and extend through seals in the envelope 11 for connection to suitable D.C. sources, not shown. Terminal wires 17 and 19 are also provided to connect the electrodes le and 16 to suitable D,C. sources, not shown.

Current for heating the cathode may be supplied to the leads 13; the cathode assembly as a whole may be maintained at a relatively high negative potential with respect to ground, while the anode, collector, and helix sections may be at substantially ground potential. This is only by way of example; it is to be understood that the various electrodes may be held at different potentials, and in some instances it may be desirable to operate with the two helix sections at different voltages.

To provide substantially reilectionless termination of the helix sections 5 and 7 on each side of the gap between them, flat radial spiral windings 2l 23 are connected to, or formed as extensions of, the helical windings S and 7, and a centrally apertured f isc E5 coated'with radio freq llency energy absorbing materialris disposed betweenV them. The envelope 11 is flared out as shown to accommodate the spirals 21 and 23 and the disc 25.

, The disc 25 may be made of insulating material, such Y Vas fused quartz, 'and the dissipative coating may be pro- V nous body ofabsorbing material, such as carbon, may be substituted for the disc 25, with its central portion cnt out like the uncoated part of the disc,25.

AAcoaxial transmission line 29 at the input end of the tube is adapted to be connected to a source, not shown, of inputsignal to be amplied. The inner conductor of the line 29 is connected to the outer end of a spiral winding 31 which, like the spirals 21 and 23 is hat and radial to the axis of the helix. The inner end of the spiral is connected to the initial end of the helix section 5.

The spiral 31 is supported between a radially dared Wall of the envelope 11 and an apertured insulating disc 33. The disc 33 is similar to the disc 25, but is not coated with dissipative material. The outer conductor of the input line 29 is connected to a centrally apertured conduc.

tive disc 35 lying outside the envelope 11 in proximity to the spiral 31.

.Y The output end of the tube has a similar arrangement for matching the end of thehelix section 7 to an output line, including a spiral'winding 31' connected between the inner conductor of the line 37 and the end-of the helix section 7. The discs 33 and 35 correspond to the discs 33 land 35 at the input end of the tube.

VThe spirals 31 and 31' provide smooth gradual transitions between the radial mode of propagation in the transmission lines 29 and 37 and the longitudinalmode in the helix. Since the transitions extend over a relatively long path, in terms of wavelength, they introduce very lit- .tle reection throughout a wide frequency band and do not require the addition of lumped reactance elements for impedance matching. At the same time, the axial distance Valong the tube occupied by the spirals is very short, enabling maximum utilization of the electron stream, and

Vpermitting the accelerator-to-helixV transit angle to be small as is required for low noise characteristics.

Similarly, the gap terminating spirals 21 and 23 provide attenuation over an extended path, as is required for broadband matching, but do not occupy very much axial space, so that the Videal effect of an insignificantly short but reilectionlessY break in the helix is substantiallyrattained.. At present, it is preferred to make theV input and output spirals of exponential pitch, with the outer turns relatively widely spaced and the space betweeny turns decreasing toward the center. However, this is not essential. The gap terminating spirals may be made of uniform pitch.

Aside from the improvements in noise gure and gain that result from the novel matchingV and terminating means, the operation of the described tube is substantially like that of known helix type travelling Wave amplifiers.

The electron stream produced by the cathode is directed along the central axis of the helix tothe collector 9. The acceleratingpotential is adjusted to make the electronsV travel at approximately the velocity of waveepropagation j on the helix. A. continuous interaction occurs between the electron stream and the wave, converting somev c-f thei pletely absorbed, and the signal is carried across solely the helix and this wave is like the original input wave but of larger amplitude. The wave builds up in the second section of the helix and is delivered by the matching spiral 31 to the output line 37. The loss in overall gain caused by the interruption of the wave on the helix is about 3,5 db.

Although the tube shown in Fig. l includes spiral matching devices at both ends as well as spiral gap terminations, it will be evident without further illustration that the improved gap terminations may be used in a tube with other types of input and output coupling means, or that theV spiral matching transitions may be used without the present gap terminations. Moreover, it is desirable in some types of travelling wave tubes to have a dissipative termination at theinput or the output end of the helix, or at both ends. For example, Fig. 3 shows the input end of the grid-input tube, in which the signal is put on the electron stream by means of a control grid 39 which produces density modulationV of the stream. Connection ofthe grid 39 to an external source, not shown, may be made throughV a metallic anged member 41 which. supports the grid assembly and may also form partof the vacuum-tight envelope. Elements of the structure shown in Fig. 3 which correspond to elements shown in Fig. 1 are referred to in Fig. 3 by primed reference numerals. Y

The helixS is terminated at its input end by a radial spiral 43, similar to the spiral 23 in Fig. l, which lies in close proximity to an energy absorbing disc 45` like the disc 25 in Fig. l. An electron gun 47 is arranged to project a beam of electrons through the control gridV 39 and down the axis ofthe helix 5 to a collector electrode, not shown. The termination comprising the spiral 43 and disc 45 serves to absorb without reilection anyV backward-travelling wave on the helix 5. The flat spiral termination permits the transit angle between the control. grid and the helix to be made small, minimizing undesir-V able longitudinal debunching ofthe electron stream.

Anothe type of travelling wave tube which may require dissipative termination at the end of the helix is that shown in Fig. 4, where a resonant cavity 49 is employedA cavity with input supplied thereto by a coaxial line 5,7.

Although the cavity 49 is itself a narrow band device, it may be adjustably tuned to any frequency throughout a substantial band by conventional means such as a plunger, not shown, and hence a broad-band termination for the helix. is desirable. Also, there maybe a tendency for standing waves to form on the helix, at frequencies independent of the cavity tuning, and it is highly desirable that these damped out. minated at'its input end by a radial spiral 43 and a dissipative disc 45, which may be identical with the correspondingly designated elements of Fig. 3.

It will be apparent that the resonator input structure shown in Fig. 4 may be used in a tube having a similar resonator at its'- output end, or in tubes using various other types of output means.

Since many changes could be made in the above con-V struction and many apparently Widely different embodiments of Vthis invention could be made without departing from the scope thereof, it is intended that all matter contained inA the abovey description or shown in the accompanying drawings shall be interpreted as illustra-` tive and not in a ylimiting sense.

, What is claimed is:

l. Ahigh frequency electron discharge device, com

prising radio frequency electromagnetic wave conductorv means, a slow wave propagating structure comprising a cylindrical helix of conductive wire, an impedance Accordingly, the helix 5 is termatching device comprising a at spiral of conductive wire adjacent one end of said helix with its inner end connected to said end of said helix and its outer en d 'connected to an end of said conductor means for providing a smooth impedance transition therebetween, said spiral being substantially coaxial with said helix and lying in a plane substantially perpendicular to the axis of said helix, and means adjacent said helix for producing and directing an electron stream along said axis through said spiral and helix for interaction with electromagnetic waves propagated along said helix.

2. A low reection termination for a slow Wave propagating structure comprising a cylindrical helix of conductive wire, said termination including a at spiral of conductive Wire adjacent one end of` said helix with its inner end connected to said end of said helix, said spiral being substantially coaxial with said helix and lying in a plane substantially perpendicular to the axis of said helix, and a body of radio frequency energy absorbing material closely adjacent said spiral and substantially coextensive therewith.

3. A high frequency electron discharge device, cornprising a cylindrical helix adapted to conduct a slowly travelling Wave, means adjacent said helix for producing and directing an electron stream through said helix in energy exchanging relationship with the travelling Wave, a coaxial transmission line for transferring radio frequency energy between said helix and an external device, and transformer means for coupling said line to said helix, said transformer means including a at spiral of conductive wire adjacent one end of said helix and substantially perpendicular to the axis thereof, with its outer end connected to the inner conductor of said line and its inner end connected to said helix.

4. A travelling wave tube including a slow wave propagating structure comprising a cylindrical helix of conductive wire, and an impedance matching termination comprising a dat spiral of conductive wire in a plane substantially perpendicular to the axis of said helix, with its inner end connected to said helix, and an attenuator element having a surface substantially parallel to the plane of said spiral and adjacent thereto, said element having radio frequency energy absorbing material distributed over said surface.

5. A gap attenuator structure for travelling Wave tubes of the type which includes a helix divided by a gap into first and second sections, including two spirals of conductive material lying respectively in substantially parallel planes on opposite sides of said gap and extending radially outward from said helix, each being connected to the adjacent end of the corresponding helix sec tion, and a body of radio frequency energy absorbing material closely adjacent said spirals and substantially coextensive therewith.

6. A gap attenuator structure for travelling wave tubes of the type which includes a helix divided by a gap into rst and second sections, said structure including two flat spirals of conductive material lying respectively in substantially parallel spaced planes, the inner end of each of said spirals being adapted to be connected to the adjacent end of a respective helix section, an element between said spirals including substantially planar surfaces closely adjacent said spirals, and radio frec-uency energy absorbing material distributed on said surfaces.

7. A travelling wave tube including a slow wave propagating structure, a transmission line for transferring electromagnetic energy between said structure and an external device, and transformer means coupling said line to said structure, said transformer means comprising a substantially flat spiral conducting member having its outer end connected to said line and its inner end connected to said wave propagating structure, said spiral conducting member having an inner opening aligned with the axis of said slow wave propagating structure, and means for producing an electron stream and projecting said stream along the axis of said slow wave propagating structure.

8. A travelling wave tube structure including a helical wave propagating conductor, means for producing an electron beam and projecting said beam along said structure, a spiral conductor for terminating said helical conductor, said spiral conductor lying in a plane substantially perpendicular to the axis of said helical conductor and having its inner end connected to an end of said helical conductor, a radio frequency energy absorbing member positioned in a plane closely adjacent to said spiral conductor, and means for modulating said electron beam at a region therealong between said beam producing means and said helical conductor.

9. The invention set forth in claim 3, wherein said spiral of conductive wire is of exponential pitch, with its outer turns relatively widely spaced and the space between turns decreasing toward the center of the spiral.

10. A high frequency electron discharge device, comprising electromagnetic wave energy conductor means, said conductor means comprising a substantially cylindrical helical portion terminated at one end by a substantially flat spiral portion having an inner opening therethrough, said spiral portion being a continuation of said helical portion with the inner opening thereof being aligned with the cylindrical opening through said helical portion, a section of transmission line conductor means having a conductor thereof comprising a continuation of the outer end of said substantially flat spiral portion, said spiral portion providing a smooth impedance transition between said cylindrical helical portion of said conductor means and said section of transmission line, and means adjacent said conductor means for producing and directing an electron stream along the axis thereof for interaction With electromagnetic waves propagated therealong.

11. A high frequency electron discharge device, cornprising travelling wave electromagnetic energy conductor means, said conductor means comprising an intermediate section of substantially cylindrical helical configuration terminated by end sections of substantially ilat spiral configuration, the axes of said end sections being aligned with the axis of said intermediate section, and means adjacent one of said end sections for producing and directing a stream of electrons along said axes through said electromagnetic energy conductor means.

12. A high frequency electron discharge device, comprising travelling Wave electromagnetic energy conductor means, said conductor means comprising a first substantially cylindrical helical section terminated at its respective ends by first and second substantially llat spiral sections, said helical and spiral sections having a first common axis, said conductor means further comprising a second substantially cylindrical helical section terminated at its respective ends by third and fourth substantially at spiral sections having a second common axis aligned with said nrst common axis, said third spiral section being closely adjacent said second spiral section, electromagnetic wave energy absorbing means located between said last-named spiral sections, and means adjacent said irst spiral section for producing and directing a stream of electrons along said first and second common axes through said electromagnetic energy conductor means.

13. A high frequency discharge device as dened in claim 12, further including electromagnetic wave energy absorbing means adjacent said first spiral section.

14. A high frequency electron discharge device, cornprising electromagnetic wave energy conductor means, said conductor means comprising a substantially cylindrical helical conductor section terminated at one end by a substantially spiral conductor section having an inner opening therethrough aligned with the opening through said helical section, at least a part of said spiral section receding from the axis of said helical section to an outer conductive portion having an appreeiably greater diameter than the diameter of said helical section, a section ofr transmission line conductor means having a conductor thereof extending from the end of Asaid outer conductive portion, said spiralrsection providing a smooth impedance transition between said cylindrical helical portion of said conductor means and said section of transmission line, and means adjacent said conductor means f for producing and directing an electron stream along the anis thereof for interaction with electromagnetic Waves conductor.

l5. VA high frequency electron discharge device comprising travelling Wave electromagnetic energy conductor means supported along an axis, said conductcr means including an intermediate section of substantially cylindrical helical configuration, aligned substantially spiral conductor'end sections terminating said conductor means, the

conductors of both endsections receding from said axis Y References Cited in the iile of this patent UNITED STATES PATENTS 1,905,353 Potter Apr. 25, 1933 1,957,538 Jensen May S, 1934 2,300,052 Lindenblad Oct. 27, 1942 2,470,307 Guanella May 17, 1949 2,516,944 Barnett Aug. 1, 1950 2,575,383 Field Nov. 20, 1951 2,578,434 Lindenblad Dec. 1l, 1951 2,584,597 Landauer Feb. 5, 1952 2,617,961 1952 Bruck Nov. 11, 

